To develop a sludge dehydrating agent which is adaptable to various kinds of dehydrators, can satisfy demand of reducing water content of dehydrated cake and also can solve the problem of increase in amount of addition of chemicals, the problem being a defect of a cross-linked or branched aqueous polymer, and which is obtained using no plyamidine-based water-soluble polymer and using (meth)acrylic monomer that is widely used as a marketed product, by developing a sludge dehydrating agent which exhibits good flocculation and high water-content-in-cake reducing ability to sewage-mixed raw sludge, sewage excess sludge, sewage digested sludge and various kinds of excess sludge in a sewage treating plant.

The problem of this invention can be attained by a flocculant containing following (A) and (B). (A); the aqueous polymer which is obtained by adding 20 to 300 ppm of a cross-linkable monomer to 80 to 100 mole of a specific cationic monomer and another monomer and a mixture of the above monomers and then by polymerizing. (B); the aqueous polymer which is obtained by adding 80 to 100 mole of a specific cationic monomer different from one in (A) and another monomer and then by polymerizing.